In structural engineering, understanding and accurately calculating wind loads is crucial for designing buildings and structures that can withstand the forces imposed by wind. The Australian Standard and New Zealand Standard AS/NZ 1170.2 (2021) provide guidelines for determining wind loads on structures.

This article aims to provide structural engineers with a detailed guide on how to calculate wind loads as per the latest standards, including a step-by-step calculation example.

Table of Contents

• Understanding AS/NZ 1170.2 (2021)
• Procedure for Calculating Wind Actions
• Key Components of Site Wind Speed Calculation
• Design Wind Speed Calculation ( V d e s , ? V_{des,\theta} Vdes,??)
• Design Wind Pressure Calculation ( p p p)
• Wind Actions ( F F F)
• Example Calculating Wind Loads By Hand
• Example Calculating Wind Loads Using ClearCalcs

Understanding AS/NZ 1170.2 (2021)

AS/NZ 1170.2 (2021) is the Australian and New Zealand Standard that specifically addresses wind loads on structures. It outlines the procedures and methodologies for determining the wind loads on various structures. The standard provides a comprehensive approach that considers factors such as terrain, topography, and the characteristics of the structure itself.

This Standard aims to provide wind actions for use in the design of structures subject to wind action. It provides a detailed procedure for determining wind actions on structures, ranging from those less sensitive to wind action to those for which dynamic responses are to be taken into consideration.

AS/NZS 1170.2:2021 is the latest iteration of this crucial standard, superseding the 2011 amendments. The 2021 revision's objectives are to remove ambiguities and incorporate recent research and experiences from recent severe wind events in Australia and New Zealand.

This Standard is Part 2 of the Structural design actions series, which comprises the following parts:

• AS/NZS 1170.0, Structural design actions, Part 0: General principles
• AS/NZS 1170.1, Structural design actions, Part 1: Permanent, imposed, and other actions
• AS/NZS 1170.2, Structural design actions, Part 2: Wind actions
• AS/NZS 1170.3, Structural design actions, Part 3: Snow and ice actions
• AS 1170.4, Structural design actions, Part 4: Earthquake actions in Australia
• NZS 1170.5, Structural design actions, Part 5: Earthquake actions � New Zealand

The Standard covers structures within the following criteria:

1. Buildings and towers less than or equal to 200 m high.
2. Structures with unsupported roof spans of less than 100 m.
3. Offshore structures within 30 km from the nearest coastline.
4. Other structures apart from: offshore structures more than 30 km from the nearest coastline, bridges, windfarm structures, and power transmission and distribution structures, including supporting towers and poles.

It's important to note that AS/NZS 1170.2:2021 is referenced by the National Construction Code (NCC) 2022, specifically within Volume 2 (Building Code of Australia). This code forms the regulatory backbone for building design and construction across Australia.

By adhering to the wind load calculation methods outlined in AS/NZS 1170.2:2021, designers and engineers ensure their projects meet the minimum safety and performance requirements stipulated by the NCC 2023. This connection emphasizes the critical link between accurate wind load assessment and compliance with national building regulations.

Procedure for Calculating Wind Actions

The procedure for determining wind actions (W) on structures and elements of structures or buildings shall be as follows:

a) Determine site wind speeds (see AS/NZS 1170.2 Clause 2.2).

The site wind speeds (Vsit,?) defined for the 8 cardinal directions (?) at the reference height (z) above ground (see Figure 2.1) shall be calculated from Equation 2.2:

V s i t , ? = V R M c M d ( M z , c a t M s M t ) V_{sit,\beta}=V_RM_cM_d(M_{z,cat}M_sM_t) Vsit,??=VR?Mc?Md?(Mz,cat?Ms?Mt?)

Where:

• V R V_R VR? = regional gust wind speed, in metres per second, for average recurrence interval of R years, as given in AS/NZS 1170.2 Section 3
• M c M_c Mc? = climate change multiplier, as given in AS/NZS 1170.2 Section 3
• M d M_d Md? = wind directional multipliers for the 8 cardinal directions (?) as given in Section 3
• M z , c a t M_{z,cat} Mz,cat? = terrain/height multiplier, as given in AS/NZS 1170.2 Section 4
• M s M_s Ms? = shielding multiplier, as given in AS/NZS 1170.2 Section 4
• M t M_t Mt? = topographic multiplier, as given in AS/NZS 1170.2 Section 4

Generally, the wind speed is determined at the average roof height (h). In some cases, this varies according to the structure.

b) Determine design wind speed from the site wind speeds (see AS/NZS 1170.2 Clause 2.3). c) Determine design wind pressures and distributed forces (see AS/NZS 1170.2 Clause 2.4). d) Calculate wind actions (see AS/NZS 1170.2 Clause 2.5).

Key Components of Site Wind Speed Calculation

Regional Wind Speed ( V R V_R VR?)

Determining the regional wind speed ( V R V_R VR?) is the first step in calculating wind load.

AS/NZ 1170.2 provides wind speed maps based on the structure's location. Engineers must identify the site's wind region and terrain category to obtain the corresponding regional wind speed.

The AS/NZS 1170.2:2021 standard classifies wind regions as cyclonic and non-cyclonic based on wind event likelihood and intensity. Cyclonic regions in Australia experience higher wind speeds and complex patterns due to tropical cyclones, while non-cyclonic regions have lower, more consistent wind speeds influenced by other meteorological systems. This classification is important for determining the appropriate design wind speed for structures in different locations.

As per Clause 3.2 of AS/NZS 1170.2, Regional wind speeds ( V R V_R VR?) for all directions based on peak gust wind data shall be as given in Table 3.1(A) or Table 3.1(B) for the regions shown in Figure 3.1(A) and Figure 3.1(B) where R R R (average recurrence interval) is the average time interval between exceedances of the wind speed listed.

In Region C, ( V R V_R VR?) values shall be obtained by linear interpolation between the value given for Region C (maximum) and the value given for Region B2 for the same R R R, according to the distance from the smoothed coastline.

In Region D, ( V R V_R VR?) values shall be obtained by linear interpolation between the value given for Region D (maximum) and the value given for Region C (maximum) for the same R R R, according to the distance from the smoothed coastline.

Figure 1: Wind regions in Australia per Figure 3.1 (A) in Clause 3.2 of AS/NZS 1170.2 (Reference)

Figure 2: Wind regions in New Zealand per Figure 3.1 (B) in Clause 3.2 of AS/NZS 1170.2 (Reference)

Wind Direction Multiplier ( M d M_d